There is a conventional liquid-crystal display device including one active device connected to an interconnection layer so as to correspond to each pixel. In this liquid-crystal display device a voltage is supplied to pixel electrodes via active devices based on the potential of the interconnection layer, whereby an electric field applied to a liquid crystal layer disposed between the pixel electrodes and a counter electrode is controlled, so that the display condition of the pixels is controlled.
In this case, when the liquid-crystal display device is a so-called active matrix type, the pixel electrodes are arranged in the form of a matrix, and a plurality of scanning lines and a plurality of signal lines positioned perpendicularly to the scanning lines independently control the display condition of each pixel region.
Three-terminal devices such as thin film transistors (TFTS) and two-terminal devices such as metal-insulator-metal (MIM) devices are known as the active devices.
In FIG. 12, the equivalent circuit of an active-matrix liquid-crystal device with a two-terminal device as an active device is shown.
MIM devices 13 as active devices are connected to a plurality of scanning lines 11 as an interconnection layer formed on the internal surface of a device substrate, and other terminals of the MIM devices 13 are connected to pixel electrodes 15. The pixel electrodes 15 are connected to a counter electrode formed on the internal surface of a counter substrate, with a liquid crystal layer 14 provided between them.
There are device capacitors C.sub.MIM in the MIM devices 13, which are connected in series to liquid-crystal capacitors C.sub.LC formed by the pixel electrodes 15, the counter electrode 16 and the liquid crystal layer 14 between them.
The MIM devices 13 are connected to the scanning lines 11. However, in reverse, there may be a case where the liquid crystal layer 14 is connected to the scanning lines 14.
In FIG. 13, a schematically perspective view of a structural example of a conventional active-matrix liquid-crystal display device is shown. In addition, in FIG. 14, a sectional view of an MIM device as one example of an active device is shown.
On the internal surface of one substrate 30A is formed a base layer 30a composed of, e.g., Ta oxide for enhancing the adhesion between the base and an interconnection layer formed thereon.
As shown in FIG. 14, on the base layer 30a is formed a first electrode portion 31a integrated with a scanning line 31 composed of, e.g., tantalum. On its surface an insulating film 33a composed of tantalum oxide is formed by using anodic oxidation. And, a second electrode portion 33b composed of, e.g., chromium is formed thereon.
The first electrode portion 31a, the insulating film 33a and the second electrode portion 33b constitute an MIM device 33.
In addition, as shown in FIG. 13, a pixel electrode 35 composed of, e.g., indium-tin-oxide (ITO) is formed to be connected to the second electrode portion 33b of the MIM device 33.
On the internal surface of another opposite substrate 30B, a counter electrode 36 composed of ITO is formed in the direction (the direction parallel to the sheet) intersecting the scanning line 31, and the counter electrode, the pixel electrode, and a liquid crystal layer provided between the substrates 30A and 30B constitute a pixel for display.
However, the liquid-crystal display device including the above conventional active device has a circuit arrangement in which the device capacitors C.sub.MIM and the liquid-crystal capacitors C.sub.LC are connected in series as shown in FIG. 12. Thus, when a potential is supplied from the scanning line 31, the device capacitor causes a decrease in the ratio C.sub.LC /(C.sub.MIM +C.sub.LC) of a voltage applied to the device capacitor of the MIM device with respect to a voltage applied between the scanning line 31 and the counter electrode 36 (signal line 32). Accordingly, problems occurs in which sufficient writing may not be performed due to the insufficient setting of the switching ratio of the MIM device, and in which voltage applied to the liquid crystal layer decreases even during a charge-holding period with the MIM device cut off after writing.
The above problems can be solved by reducing the device capacitance of the MIM device if possible. Reducing the device capacitance requires, for example, a reduction in the device area. However, it is difficult in production to form the device smaller than the conventional one.
Accordingly, the present invention solves the above problems, and provides a novel structure for ensuring sufficient a writing operation without decreasing a voltage applied to liquid crystal by different means from a method for improving the device characteristics of an active device such as an MIM device.